Metered combustion air supply system



' Dec. 27, 1966 s. VOORHEIS- METERED COMBUSTION AIR SUPPLY SYSTEM Filed April 15. 1965 R O T N F V W TEMPLE 8. VOORHEIS ATTORNEYS United States Patent 3,294,146 METERED COMBUSTION AIR SUPPLY SYSTEM Temple S. Voorheis, Palo Alto, Calif., assignor to Coen Company, Burlingame, Calif., a corporation of California Filed Apr. 15, 1965, Ser. No. 448,304 2 Claims. (Cl. 158-1) This invention relates to a system for supplying combustion air to a burner of the type used in firing boilers and the like, and more particularly to a method for accurately regulating air flow to such burner over a wide range of load conditions.

Burners of the type used in commercial and marine boilers and the like are preferably adjustable over a broad range in order to accommodate varying load demands on the burner and on the boiler that it fires. Such burners must be adapted to supply air and fuel, either gaseous or liquid, in proportionate amounts necessary to supply the load and to provide optimum combustion. In many burner installations, a blower or fan for producing a forced air draft is located remote from the combustion chamber because of space requirements adjacent the combustion chamber or because of the desirability of avoiding placement of noisy machinery in the furnace or boiler room. Air ducts extending to the burner from a remotely located blower or fan present problems of metering or regulating air flow not present when the blower is immediately adjacent the burner. For example, the inner walls of air ducts accumulate dust, dirt, soot and the like with the passage of time which accumulation changes the flow characteristics through the duct.

Prior art air control systems of which I have knowledge can be classified in two categories. The first category includes air control systems wherein a damper or like air throttling structure is mechanically positioned according to the position of a fuel valve. Reference is made to my US. Patent No. 2,818,109 for an example of such system. The second category includes systems in which a differential pressure sensor is arranged to sense the pressure drop across an orifice in the air flow system and to maintain such differential at a magnitude proportional to the position of a fuel control valve.

Systems in the first category, although affording a Wide range of operation, are insensitive to changing air supply pressure conditions arising from accumulation of soot and the like on interior duct and damper walls. The systems of this category are likewise insensitive to variation in draft conditions arising from soot accumulations on combustion chamber walls and stackwalls and from varying wind conditions at the stack. Consequently, although prior art systems in this category are reasonably accurate when first installed and set up, such accuracy decreases with time.

The second category of prior art systemsv with which I am familiar obviates many of the aforementioned disadvantages but do not afford sufficient range of air flow variation. Such limitation in air flow control range arises from the fact that the differential pressure acrossan orifice varies as the square of the rate of air flow through the orifice. By way of illustration, a to 1 reduction in air flow requires a 100 to 1 reduction in differential pressure. Differential pressure controlling systems capable of a 100 to 1 control variation, if available, are extremely complex and expensive.

The principal object of the present invention is elimination of the above mentioned shortcomings and disadvantages existent in prior art systems of this type. Such is accomplished in the present invention by providing an air damper adjacent to and in series with a burner throat opening of a combustion chamber, by measuring the pressure differential across the series combination of these 3,294,146 Patented Dec. 27, 1966 elements, and by maintaining such pressure differential at a constant value. Accordingly, the rate of air inflow is proportional to the position of the air damper. Restrictions upstream or'downstreamof the series combination will not affect the rate of air flow therethrough so long as the differential pressure thereacross is maintained constant. A second damper upstream of the first damper is provided and is continuously regulated so as to maintain constant the differential pressure across. the series combination. The first damper is positioned according to rate of fuel inflow. The second, or upstream, damper is continuously and automatically regulated to maintain pressure differential across the first damper at a constant value. Thus, air flow is proportional to the position of the first damper, thereby providing a broad operating range because the first damper can be closed almost completely. Accuracy of air inflow rate is secured entirely independently of restrictions upstream of the first damper or downstream of the combustion chamber burner throat.

Another object is to provide a unique air flow control structure that acts as a variable area orifice so that total flow therethrough can be varied by maintaining the differential pressure across the orifice at a constant rate and by varying the effective cross-sectional area of the orifice. Achievement of this object avoids problem caused by the aforementioned square law relationship between flow rate and differential pressure.

Still another object of the present invention is to provide necessary control systems for effecting the mode of operation to which reference has been made hereinabove.

A feature and advantage of the present invention is that a useful operating range in excess of ten to one is attained since the system affords accurate air flow control even at greatly reduced rates of operation.

Another feature and advantage is that even at low rates of operation, sufficient air is provided to sustain combustion and avoid inadvertent attempts to operate the burner with insufficient air.

Other objects, features and advantages will be more apparent after referring to the following specification and the accompanying drawing in which:

FIG. 1 is a partially schematic view of an air supply system according to the present invention;

FIG. 2 is a cross-sectional fragmentary view of a combustion chamber burner throat; and

FIG. 3 is a schematic view of differential pressure control element of the present invention.

Referring more particularly to the drawings, reference numeral 12 indicates a fan for producing a forced air draft in a duct 14 mounted in air communication with the outlet side of the fan. The opposite end of duct 14 is mounted in air communication with a plenum chamber or windbox 16 which is affixed to a furnace or combustion chamber wall 18 and in air communication with a refractory burner throat 20 formed through the wall. It will be seen that the air draft produced by fan 12 passes through duct 14, plenum chamber 16, and throat 20 into the interior of the combustion chamber. liquid or gaseous form, is delivered through a nozzle structure 22 into the combustion chamber for combustion in the presence of air supplied as described above.

Mounted adjacent the junction of duct 14 and plenum chamber 16, i.e., at the inlet side of the plenum chamher, is a damper indicated generally at 24, of substantially greater cross-sectional area than burner throat 20, as shown in FIG. 1. The damper is composed of a plurality of transversely-extending pivotally-mounted vanes 26, the positions of which are controlled in unison by conventional linkage through a lever arm 28, so as to control the rate of air flow into plenum chamber 16.

Within plenum chamber 16 and concentrically of burner throat 20 can be provided any suitable form of air Fuel, in either a 3 distributing structure 30 an exemplary form of which is shown in my US. Patent No. 2,818,109. Such air distributors typically cause insignificant pressure drop.

The series combination of damper 24 and burner throat 20 form a restriction or orifice through which air flows from duct .14 into the combustion chamber. A pressure drop occurs across such restriction. By maintaining such pressure drop at a constant magnitude, the rate of air flow into combustion chamber is made proportional to the pivotal position of vanes 26 in damper 24. The position of the vanes is controlled by a control shaft 36 to which is also operatively connected a connecting rod 37 for controlling rate of fuel flow through nozzle 22. It will be seen that adjustment of control shaft 36 will permit variation in the load and maintain the proportions of fuel and combustion air in proper relation.

In order that the rate of air inflow be proportional to the position of vanes 26 in the windbox inlet damper, the pressure across the damper and the burner throat 20 is maintained constant. For this purpose, a pressure tap 38 is provided in duct 14 adjacent to an upstream of damper 24. A second pressure tap 40 is disposed within the combustion chamber, that is, downstream of burner throat 20, as a consequence of which the pressure differential across the orifice formed by the damper and the burner throat is sensed by the differential of the pressure existing at taps 38 and 40. A signal proportional to such differential pressure is derived by a sensor 42 which is fed back for employment in adjusting the air flow through duct 14 so as to maintain the pressure differential across damper 24 and burner throat 20 at a constant magnitude so that air inflow rate is proportional to the position of vanes 26.

Exemplary of devices for controlling air flow delivered by duct 14 to windbox 16 is a vortex damper 44 mounted at the inlet port of fan 12. The vortex damper controls lair delivered to plenum chamber 16 by throttling the amount of air drawn in the fan. Such damper is typically provided with an operating lever 46 which is operatively connected so as to open and close the damper.

Vortex damper 44 is to be considered exemplary of any suitable air control means upstream of pressure tap. A full equivalent of such damper for purposes of this invention is an adjustable damper at the discharge side of fan 12 and upstream of pressure tap 38. So long as means are provided for adjustably controlling the rate of air inflow delivered at the inlet side of damper 24 in response to the pressure differential across the damper and burner throat 20, practice of the invention is possible.

Operatively connected to lever 46 is a first or opening motor 48 and a second or closing motor 50. A conductor 52 is provided for connecting motor 48 to sensor 42; a conductor 54 is provided for connecting motor 50 to the sensor. Referring to FIG. 3, sensor 42 includes an airtight chamber 56 in which is mounted a diaphragm 58 that divides the chamber into two compartments. Pressure tap 38 is mounted in air communication with one of the compartments; pressure tap 40 is mounted in air communication with the other compartment. It will be seen, therefore, that the position of diaphragm 58 corresponds to the differential pressure between a point upstream of damper 24 and a point with: in the combustion chamber. Affixed centrally of diaphragm 58 is an operating rod 60 that is carried by a pantograph linkage 62. A force is imposed on rod 60' axially thereof by a compression spring 64 acting on pantograph linkage 62 through a simple lever system 66. Such force on diaphragm 58 retains the diaphragm in a central position within chamber 56 when the pressure diftferential across damper 24 and burner throat 20 is at a preselected desired magnitude. In such condition, an armature 68 carried by rod 60 is positioned midway between confronting electrical contacts 70 and 72.

Electrically connected to armature 68 is a power source,

4- indicated schematically at 74, for driving motors 48 and 50. Closure of a circuit through the armature and contact 70 energizes motor 48 to open damper 44 and thereby to increase the air flow through windbox 1 6. Closure of a circuit between the armature 68 and contact 72 energizes motor 50 to close damper 44.

An unwanted increase in rate of air flow through damper 24 and burner throat 20, caused for example by varying wind currents at the stack of the furnace, will cause an increase in the pressure differential across the damper and throat. Such increase is applied through pressure taps 38 and 40 to diaphragm 58 which moves rod 60 rightwardly, as viewed in FIG. '3, and causes activation of motor 50 through armature 68 and contact 72. Motor 50 causes closure of damper 44 and reduces the pressure differential, and therefore the rate of air flow, to a preselected desired value. Obviously, should the differential pressure fall below a preselected desired value, diaphragm 58 and rod 60 will move leftwardly, as viewed in FIG. 3, in response to the force of spring 64. Such action will close a circuit from power source 74 to motor 48 thereby opening damper 44 to increase the air flow rate.

The operation of the system may be understood by assuming that shaft 36 has been set in accordance with a required load demand in the combustion chamber. Such adjustment of the shaft fixes the position of vanes 26 in damper 24 and the rate of fuel inflow through line 22. The volume of air entering the combustion chamber is therefore proportional to the pressure drop across the damper and throat and will be constant if such pressure differential is held constant. Pressure taps 38 and 40 in cooperation with sensor 42 sense such pressure differential and through motors 48 and 50 regulate the position of damper 44 so as to maintain the pressure differential at a constant magnitude. Any restrictions occurring at fan 12, duct 14, or the combustion chamber stack will be automatically compensated for by this system to the end that an optimum amount of combustion air is always present.

When it is desired to adjust the combustion rate 7 within the combustion chamber, for example, to reduce such rate, shaft 36 is positioned to close vanes 26 of damper 24. The effective cross-sectional area available for transpiration of air is thereby reduced. Since sensor 42 in cooperation with motors 48 and 50 and damper 44 act to maintain the pressure differential across the orifice constant, the rate of air flow into burner throat 20 is reduced in correspondence with the amount of closure of damper 24. Consequently, the rate of combustion is reduced by concurrent reduction of both air and fuel flow in response to movement of shaft 36.

In the drawing, duct 14 is shown with a section thereof in broken lines. Such showing is for the purpose of illustrating that the present control system permits fan 12 to be located at any mechanically convenient site without sacrificing precise control of the rate of air,

flow into the combustion chamber. Consequently, prior art systems requiring complex mechanical linkages from the burner to remotely located fans for adjusting the position thereof are eliminated. Moreover, the shortcomings and inaccuracies inherent in prior art systems of the type relying solely on manual positioning of dampers are eliminated.

An important feature and advantage of the present invention can be appreciated by considering the air supply system installed in a burner used for firing a steam boiler and wherein it is desired to provide steam at a preselected variable pressure. Typical in such boiler system is a pressure sensitive controller 76 that is coupled to a point in the steam system through a pressure line 7 8. Controller 76 has an output lever 80 and acts, in a conventional way, to move lever 80 to a position proportional to the pressure existent in line 78. Linkage 82 is coupled from. lever 80 to control shaft 36 so that the position of the shaft is proportional to pressure in the steam boiler being fired by the burner of the present invention. The controller is arranged to respond when the steam pressure drops to an unsuitably low level so as to cause actuator 76 through linkage 82 and shaft 36 to open damper 24 and the fuel valve in line 2-2. This causes the rate of combustion to increase which in turn causes an increase in steam pressure which is sensed by controller 76 to throttle the air and fuel supply when the low steam pressure condition is corrected. It will be appreciated that the present air supply system enables boilers of the type described to be controlled automatically.

It will thus be seen that the present invention provides a burner control system capable of a precise and widely variable mode of operation not possible in prior art structures of which I am aware. Moreover, because dilferential pressure sensor 42 establishes a feedback loop from the burner to the upstream damper, adequate amounts of air for full combustion are always available.

Although one embodiment of the present invention has been shown and described, it will .be apparent that other adaptations and modifications can be made without departing from the true spirit and scope of the invention.

What is claimed is:

1. In combination: a single burner having a fixed inlet throat opening; a forced air draft source for supplying air to said burner through said throat opening; a wind box connecting said source to said burner; primary air control means in an open inlet side of said wind box between said source and burner and being in series combination with said throat opening; said primary air control means comprising at least one adjustable vane movable from a substantially fully open position to a nearly closed position to control the rate of air fiow from the air source to the burner in relation to the operating load demand of said burner; the cross sectional area opening of the primary damper when in substantially open position being substantially larger than the fixed cross sectional area of the throat opening, and the cross sectional area opening of the primary damper when the vanes are in nearly closed position being substantially smaller than the cross sectional area of the throat opening, whereby the air pressure drop across the primary air control means when the latter is in nearly closed position is substantially greater than the pressure drop across the throat opening, and whereby the pressure drop across the throat opening is substantially greater than the pressure drop across the primary air control means when the latter is in substantially open position; secondary adjustable air control means located to the upstream side of said primary air control means operable to control the rate of air flow from the source to the burner; pressure sensing means for sensing pressure differential across the series combination constituted by said primary damper and said throat opening; and means operatively connected to and responsive to said pressure sensing means for adjusting said secondary air control means to maintain a substantially constant predetermined pressure differential across the series combination.

2. The invention of claim 1 wherein said secondary air control means comprises a damper having at least one vane and wherein said adjusting means includes means linked to said secondary air control means for positioning said damper.

References Cited by the Examiner UNITED STATES PATENTS 1,620,240 3/1927 Smoot. 2,163,592 6/1939 Dickey 158-1 2,49 9,076 2/ 0 Rosenberger. 2,818,109 12/1957 Voorheis 158-105 JAMES W. WESTHAVER, Primary Examiner. 

1. IN COMBINATION: A SINGLE BURNER HAVING A FIXED INLET THROAT OPENING; A FORCED AIR DRAFT SOURCE FOR SUPPLYING AIR TO SAID BURNER THROUGH SAID THROAT OPENING; A WING BOX CONNECTING SAID SOURCE TO SAID BURNER; PRIMARY AIR CONTROL MEANS IN AN OPEN INLET SIDE OF SAID WIND BOX BETWEEN SAID SOURCE AND BURNER AND BEING IN SERIES COMBINATION WITH SAID THROAT OPENING; SAID PRIMARY AIR CONTROL MEANS COMPRISING AT LEAST ONE ADJUSTABLE VANE MOVABLE FROM A SUBSTANTIALLY FULLY OPEN POSITION TO A NEARLY CLOSED POSITION TO CONTROL THE RATE OF AIR FLOW FROM THE AIR SOURCE TO THE BURNER IN RELATION TO THE OPERATING LOAD DEMAND OF SAID BURNER; THE CROSS SECTIONAL AREA OPENING OF THE PRIMARY DAMPER WHEN IN SUBSTANTIALLY OPEN POSITION BEING SUBSTANTIALLY LARGER THAN THE FIXED CROSS SECTIONAL AREA OF THE THROAT OPENING, AND THE CROSS SECTIONAL AREA OPENING OF THE PRIMARY DAMPER WHEN THE VANES ARE IN NEARLY CLOSED POSITION BEING SUBSTANTIALLY SMALLER THAN THE CROSS SECTIONAL AREA OF THE THROAT OPENING, WHEREBY THE AIR PRESSURE DROP ACROSS THE PRIMARY AIR CONTROL MEANS WHEN THE LATTER IS IN NEARLY CLOSED POSITION IS SUBSTANTIALLY GREATER THAN THE PRESSURE DROP ACROSS THE THROAT OPENING, AND WHEREBY THE PRESSURE DROP ACROSS THE THROAT OPENING IS SUBSTANTIALLY GREATER THAN THE PRESSURE DROP ACROSS THE PRIMARY AIR CONTROL MEANS WHEN THE LATTER IS IN SUBSTANTIALLY OPEN POSITION; SECONDARY ADJUSTABLE AIR CONTROL MEANS LOCATED TO THE UPSTREAM SIDE OF SAID PRIMARY AIR CONTROL MEANS OPERABLE TO CONTROL THE RATE OF AIR FLOW FROM THE SOURCE TO THE BURNER; PRESSURE SENSING MEANS FOR SENSING PRESSURE DIFFERENTIAL ACROSS THE SERIES COMBINATION CONSTITUTED BY SAID PRIMARY DAMPER AND SAID THROAT OPENING; AND MEANS OPERATIVELY CONNECTED TO AND RESPONSIVE TO SAID PRESSURE SENSING MEANS FOR ADJUSTING SAID SECONDARY AIR CONTROL MEANS TO MAINTAIN A SUBSTANTIALLY CONSTANT PREDETERMINED PRESSURE DIFFERENTIAL ACROSS THE SERIES COMBINATION. 